TJBS diodes offer greater flexibility for component design and are suitable, in particular, as Zener power diodes having a breakdown voltage of approximately 20V for use in motor vehicle generator systems.
More and more functions in modern motor vehicles are being performed by electrical components. This results in an ever higher demand for electrical power. To meet this demand, the efficiency of the generator system in the motor vehicle must be increased.
Until now, p-n type diodes have ordinarily been used as Zener diodes in motor vehicle generator systems. The advantages of p-n type diodes are a low reverse current, on the one hand, and high robustness, on the other hand. The main disadvantage is a high forward voltage UF. At room temperature, current begins to flow only at UF=0.7V. Under normal operating conditions, e.g., a current density of 500 A/cm2, UF increases to over 1V, which means a not inconsiderable loss in efficiency.
Theoretically, Schottky diodes are available as an alternative. Schottky diodes have a much lower forward voltage than p-n type diodes, for example 0.5V to 0.6V at a high current density of 500 A/cm2. As majority carrier components, Schottky diodes also have advantages in rapid switching operation. However, Schottky diodes have not yet been used in motor vehicle generator systems. This circumstance is attributable to a number of important disadvantages of Schottky diodes: 1) higher reverse current compared to p-n type diodes; 2) great dependency of the reverse current on reverse voltage; and 3) poor robustness, in particular at high temperatures.
Ways to improve Schottky diodes have already been proposed. Two examples are listed below. JBS (junction barrier Schottky diode), which is described in S. Kunori, et al., “Low leakage current Schottky barrier diode,” Proceedings of 1992 International Symposium on Power Semiconductors & ICs, Tokyo, pp. 80-85.
As shown in FIG. 1, the JBS includes an n+-type substrate 1, an n-type epilayer 2, at least two p-type wells 3, which are diffused into n-type epilayer 2, and metal layers on front 4 and back 5 of the chip. From an electrical point of view, the JBS is a combination of a p-n type diode having a p-n junction between p-type wells 3 as the anode and n-type epilayer 2 as the cathode, and a Schottky diode having a Schottky barrier between metal layer 4 as the anode and n-type epilayer 2 as the cathode. The metal layer on the back of chip 5 acts as a cathode electrode, while the metal layer on the front of chip 4 acts as an anode electrode having an ohmic contact to p-type wells 3 and, at the same time, as a Schottky contact to n-type epilayer 2.
Due to the low forward voltage of the Schottky diode, compared to the p-n type diode, currents flow only through the area of the Schottky diode in the forward direction. As a result, the effective area (per unit of area) for the flow of current in the forward direction is much smaller in a JBS than in a conventional planar Schottky diode.
In the reverse direction, the space charge zones expand as the voltage increases and converge in the middle of the area between adjacent p-type wells 3 at a voltage which is lower than the breakdown voltage of the JBS. This partially shields the Schottky effect, which is responsible for the high reverse currents, and reduces the reverse current. This shielding effect is highly dependent on the structural parameters of penetration depth of p-type diffusion Xjp, the distance between p-type wells Wn and the width of p-type well Wp as well as on the doping concentrations of p-type well 3 and n-type epilayer 2 (see FIG. 1).
Conventionally, the p-type wells of a JBS are implemented by p-type implantation followed by p-type diffusion. Lateral diffusion in the x direction, whose depth is comparable to the vertical diffusion in the y direction, results in cylindrical p-type wells in the two-dimensional representation (infinite length in the z direction perpendicular to the x-y plane), whose radius corresponds to penetration depth Xjp. Due to the radial expansion of the space charge zones, this form of p-type wells does not demonstrate a very effective shielding of the Schottky effect. It is not possible to strengthen the shielding effect solely with the aid of a deeper p-type diffusion, since the lateral diffusion becomes correspondingly wider at the same time. Reducing the distance between p-type wells Wn is also not a good approach, since, while this strengthens the shielding effect, it also further reduces the effective area for current flow in the forward direction.
An alternative for improving the shielding action of the Schottky effect or the barrier-lowering effect of a JBS is described in German Patent Application No. DE 10 2004 053 761 and referred to as TJBS. FIG. 2 shows a TJBS (trench junction barrier Schottky diode) of this type, which has filled-in trenches. The TJBS includes an n+-type substrate 1, an n-type epilayer 2, at least two trenches 6, which are etched into n-type epilayer 2, and metal layers on the front of chip 4 as the anode electrode and on the back of chip 5 as the cathode electrode. The trenches are filled with p-doped Si or poly-Si 7. In particular, metal layer 4 may also include multiple different metal layers which lie on top of each other. For the sake of clarity, this configuration is not plotted in FIG. 2. From an electrical point of view, the TJBS is a combination of a p-n type diode (p-n junction between p-doped trenches 7 as the anode and n-type epilayer 2 as the cathode), and a Schottky diode (Schottky barrier between metal layer 4 as the anode and n-type epilayer 2 as the cathode).
As in a conventional JBS, currents flow only through the Schottky diode in the forward direction. Due to the lack of a lateral p-type diffusion, however, the effective area for current flow in the forward direction is much larger in the TJBS than in a conventional JBS. In the reverse direction, the space charge zones expand as the voltage increases and converge in the middle of the area between adjacent trenches 6 at a voltage which is lower than the breakdown voltage of the TJBS. As in the JBS, this shields the Schottky effect, which is responsible for high reverse currents, and reduces the reverse currents. This shielding effect is greatly dependent on the structural parameters of trench depth Dt, the distance between trenches Wm and trench width Wt as well as on the doping concentrations of p-type well 7 and n-type epilayer 2 (see FIG. 2).
No p-type diffusion is used to provide the trenches in the TJBS. As a result, there is no negative effect of lateral p-type diffusion, as in a conventional JBS. An approximately one-dimensional expansion of the space charge zones in the mesa area between trenches 6 may be easily implemented, since trench depth Dt, which is an important structural parameter for shielding the Schottky effect, no longer correlates with the effective area for current flow in the forward direction. The shielding action of Schottky effects is thus much more effective than in the JBS having diffused p-type wells.
On the other hand, the TJBS has a high degree of robustness, due to its clamping function. The breakdown voltage of p-n type diode BV_pn is designed in such a way that BV_pn is lower than the breakdown voltage of Schottky diode BV_schottky, and breakdown occurs at the bottom of the trenches. In breakdown mode, the reverse current then flows only through the p-n junction. The forward direction and the reverse direction are thus geometrically separated. The TJBS thus has a robustness which is similar to that of a p-n type diode. In addition, the injection of “hot” charge carriers in oxides does not occur in a TJBS, since no MOS structure exists. Consequently, the TJBS is suitable as a Zener diode for use in motor vehicle generator systems.